There is known a sheet processing apparatus including a detection unit for detecting information for determining the type or authenticity of sheet or information for finding a contamination or damage of a sheet, and directed for eliminating a contaminated or damaged sheet and classifying sheets by the type. Authenticity of a sheet is determined based on a detection result from a magnetic sensor for detecting magnetism of the sheet.
However, when a machine for conveying a sheet is arranged near the magnetic sensor and magnetism of a sheet is detected by the contactless type magnetic sensor, there is a problem that a noise other than magnetism of a sheet is detected and an accuracy of detecting magnetism of a sheet is deteriorated.